Just to chime in, 0AD is indeed not trying to be sexist or anything, but... Most of the civilizations portrayed were essentially sexist (explicitly patriarchal, with strong and rigid gender-roles). They were also mostly genocidal bigots... The ancient world was a little rough on the edges... Most of the ancient armies were close to 100% male, so in a game of historical warfare, it's relatively normal to have such an overrepresentation of men. That doesn't mean it wouldn't be cool to have more female representation, but as a game that strives for historical accuracy, women in military roles need to be well researched. As mentioned, the Mauryas have Maiden Guards, the Britons have Boudica, the Kushites have Amanirenas. In the mod Millennium AD, the Norse have shield maidens. I'm personally excited about a Scythian civ, as are others. Scythians would have plenty of female fighters (up to 1/3 of their forces), and lets not forget Queen Tomyris. It's a very civ-specific question... I believe Thracians might have had some female warriors, but I'd have to read up more on it.
That having said, Celtic women were definitely more engaged in military affairs than Roman or Greek women for example... Having Briton/Gallic women being able to build military structures doesn't actually sound totally terrible to me... It's an interesting idea for a civ-specific bonus.
Questions in ethics are always good to ponder on, but are a little awkward in a game where you're tasked with wiping out all that oppose your domination of the map. You can realistically kill up to several thousand people in a match, putting games like GTA to shame...
On a personal note I would actually like to see male and females combined in an economic villager unit without specific gender-roles.